Gain controlled telegraph receiver



Mai 19 11942 i R. A. woon 2,283,404

GAIN CON'IROIJLED4 TELEGRAPH RECEIVER' y Fil'ed June 28, 1959 INSTRUMENT ro/vs asf/ERA roe ATTORNEY Patented May 19, 1942 UNITED STATES GAIN CONTROLLED TELEGRAPH RECEIVER Raymond A. WoodLSanta Ana, Calif.,assignor to Radio Corporation of America, a corporation of Delaware Application June 2s, 1939, serial No. 281,629

5 Claims.

This invention relates to gain controlled telegraph receivers and more particularly to an arrangement suitable for the reception of keyed telegraph signals.

It is an object of my invention to provide apparatus including an automatic gain control system, a limiter device, and a tone generator for impressing tone modulations on the keyed signals, all combined for keyed telegraph signal reception, Y

It is another object of my invention to provide a system o-f the class indicated above wherein smooth operation of the automatic gain control in an autodyne receiver is obtained.

vIt is another object of my invention to piovide a system wherein a constant biasing voltage is obtained during periods of no signal.

It is a further object of my invention to prevent the biasing voltage in an automatic gain control device from falling to such a low value that upon initiation of a marking signal the autodyne detector might be adversely drawn into a zero beat condition with respect to the frequency of the incoming signal.

It is another object of my invention to provide a telegraph receiver in which the effects of noise and atmospheric disturbances shall be reduced to a minimum, especially during no-signal periods.

Other objects and advantages of my invention will lbe made manifest upon reading the detailed description to follow, and when considered in View of the accompanying drawing, the sole iigure of which shows diagrammatically a preferred circuit arrangement Briefly stated, the invention comprises a radio receiver having autodyne detection and an automatic volume control device, a limiter stage, and an output stage on which tone frequency oscillations are impressed for delivering tone modulated marking signalsto any suitable telegraph instrument.

The preferred circuit arrangement will nowk be described in detail with reference to the accompanying drawing.

lThe receiving antenna I8 feeds radiant energy as collected thereby into a radio receiver I9 Vthe elements of which are more or less conventional and have not been shown in detail. It will be understood, however, that the signal after amplification may be impressed upon a transformer 20, the secondary of which is divided, the two terminals leading respectively to different anodes in a double diode detector tube 2|. The midpoint of the secondary winding of 55 the transformer 20 is connected to one terminal of a resistor 5 which is in series with another resistor 4 leading to ground. The cathode of the tube 2I is also grounded'. By virtue of this arrangement the detection of a marking signal results in the development of a potential drop across the resistors 5 and 4, the point D being rendered negativev with respect to' ground.

A portion of the direct current routput energy from the receiver is applied to an automatic gain control device which includes the discharge tubes 2 and 3. The interconnection between resistors 4 and 5 is connected to the cathode of tube 2 and to the anode of tube 3. These tube electrodes are, therefore, maintained at virtually the same potential which is the potential at the 'point A, this being more or less negative with respect to ground, depending on the potential drop across resistor 4. This potential drop is a direct function ofthe rectied signal strength. The anode of tube 2 is connected through junction B to one side of a capacitor I, the other side being grounded If a strongly negative voltage is developed at point A during a key down?` or marking period, then the potential drop in the resistor 4 will cause tube 2 to draw current, assuming that its anode (connected to the upper electrode of the condenser I through junction B) is at the moment maintained at' a relatively positive potential. This condition will occur at any time when the charge stored'on capacitor I is less than the potential which is represented .by the drop in resistor 4. If, however, the potential stored in capacitor I is greater than the potential drop developed in resistor 4 by the rectification of signal energy, then the potential at junctionB will be more negative than atpoint A and tube 3 will be rendered conductive. Tube 3, however, is controlled by the operation of a limiter tube l and during spacing intervals is biased to cut- 01T.

It will be seen from the above explanation that capacitor I is arranged to vstore a variable charge dependent upon the strength of the marking signals and this charge is availablefor the operation of the automatic'volume control circuit 22. During spacing intervals, however, neither of the tubes 2 and 3 will be conductive and the value xed during a marking period will be that which persists during a subsequent spacing period insofar as the bias of the AVC circuit is concerned. During key-up or spacing in- .tervals the voltage developed at th'e point D is raised to a value approximate 'to ground 'potential. The control grid of a limiter tube 1 is, therefore, raised to a point about the cut-off potential and this tube, which, therefore, draws current from the direct current source 8. A voltage is thereby developed across the resistor 9, one end of which is grounded, as is the cathode of the tube 1. A resistor 6 of high ohmic value is connected between the grid of tube 3 and the output circuit of tube 1 adjacent the negative terminal of the direct current source 8. When the limiter tube 1 draws current, then the grid of tube 3 assumes a negative potential, thereby blocking this tube. Also during key-up? intervals the voltage at the point D, and consequently at point A, is practically zero, so, if the capacitor I'at that time produces a negative potential at the point B, no current can iiow through tube 2, that being a unilateral conductance. Thus capacitor I will hold its charge during key-up intervals. l

During key-down intervals tube 1 is cut off so that thepotential on the grid of tube 3 becomes zero and this tube is rendered conductive of a potential in the direction of anode-tocathode. If the signal is so strong that point A b'ecomes more negative than point B, current will flow throughtube 2. Thus point B will in either case assume a potential substantially the same as that of point A during each key-down interval and will hold its potential substantially at the last established value during key-up periods.

A function of the limiter tube 1 is to key a locally generated tone which is derived from a suitable low frequency generator I2. The output from this tone generator is applied to the primary of a transformer I3 having a center-tapped secondary. A push-pull discharge tube arrangement consisting of two tubes I4, or their equivalent, is controlled by potentials developed across the two halves of thesecondary in transformer I3. The mid-point of this secondary is connected to the negative side of the source 8 and is thus controlled by the limiter Vtube y1 in accordance with its alternately conductive and non-conductive conditions and in response to key-up. and key-down intervals respectively. It will thus be seen that the push-pull tubes I4 are enabled .to deliver tone modulated keyed signals in response to the radiant energy signals as collected on the antenna IB. The output from the tubes I4 is delivered to the primary of a transformer I6. A mid-point on this primary is connected to the positive side of a direct current source I5, the negative terminal of which is connected to the grounded cathodes of the tubes I4.

A detailed description of the mode of operation of the device has been given along with a recital of theelements in combination and of the circuits by which these elements are interconnected. A more comprehensive idea of the advantages to be derived from the circuit arrangement as herein presented may be gained from the following account of actual performance of the device itself. 4

It is found in practicing the invention that the biasing voltage applied to the circuit 22 by virtue of the regulation of charge on the capacitor I is such as to obtain a smooth flowing output from the receiver. The capacitor I, as has been explained hereinbefore, is subject to the application thereto of a variable charge dependent upon the strength of the rectified signals. This charge ductive. `I`t can also be decreased `by rendering the tube 3 conductive, but in either case the adjustrnent of the charge on the capacitor I is made during the lapse of a marking element of the signal and is prevented from being altered during the lapse of a spacing signal because tube 3 is blocked by the conductive state of the limiter tube 1 during spacing signals. The limiter tube 1 is controlled by the key-up and keydown conditions of reception as detected in the tube 2l of the receiver. The action of the tube 1 is such as to key the tone frequency output from the generator I2 and to impress these tone modulated key-signals upon the push-pull amplifier can be increased by rendering the tube 2 con- 75 tubes I4. The output transformer I6 has a secondary winding which is connected directly to any suitable telegraph instrument. The strength of the signal applied to the limiter tube 1 may be suitably adjusted by shifting the tap 23 on the potentiometer 24 which connects between the point D and ground. The resistor 6 is one of very high value, thus preventing an unfavorable interaction between the capacitor I and the keyer tube 1. The resistors 4 and 5 are preferably adjusted to maintain approximately one-quarter of the total voltage set up for biasing. This adjustment provides very smooth control of the receiver and automatically readjusts the biasing voltage during each keying or marking pulse.

It has been found in practicing the invention that the system as herein disclosed tends to eliminate noise and atmospheric disturbances and prevents to a large extent the transmission of erroneous signals to the telegraph instrument. It has been observed that during idle periods, or key-up intervals, the telegraph instrument is rendered substantially unresponsive to disturbing signals.

Various modifications of my invention may be made by those skilled in the art, merely by applying the principles herein set forth. Furthermore the embodiment herein illustrated maybe adapted to a variety of situations under which the invention is toV be practiced. The scope of the invention is, therefore, limited only in accordance with the claims. Y

I claim: I

l. Telegraph apparatus comprising a receiver having a double diode detector, a signal responsive circuit for controlling the gain in said receiver, said circuit including vmeans for deriving potentials from said detector during reception of intermittent marking signals, a capacitor having one electrode operatively connected to said circuit and the other electrode grounded, means including a unilateral conductance for feeding said potentials to said capacitor and for storing a charge therein, and means including a second unilateral conductance for at times permitting the reduction of said charge; an amplitude limiter device under control of saidv potentials, and means operable by said amplitude limiter for regulating the impedance value of said second unilateral conductance. A

2. Apparatus for the suppression of noise effects especially during spacing intervals between the marking elements of telegraph code signals, said apparatus comprising a radio receiver having an automatic gain control circuit subject to a variable biasing potential, means responsive to the reception of a marking signal for storing said biasing potential during a subsequentv spacing interval, thereby to reduce the gain in said receiver, means including a limiter discharge tube and an amplifier tube operative therefrom for at times increasing the gain in said receiver, and means including a push-pull amplifier under oontrol of a tone frequency generator and under iurther control of said limiter discharge tube, for deriving keyed tone signals corresponding to said `marking elements of the received code signals.

3, Apparatus for the suppression of noise effects comprising a receiver responsive to code signals composed of marking and spacing elements, means for rectiiying the energy of. the marking elements, means for storing said energy during spacing elements, an automatic volume control c-ircuit effective to control the gain in said receiver, an amplitude limiter device arranged to be blocked during reception of marking signals, and two unilateral conductances arranged as means for maintaining a given Value of bias control potential on said circuit during the lapse of each spacing element and for adjusting said Value only during the lapse of each marking element, one of said unilateral conductances being subject to control by potentials `derived from said amplitude limiter device.

4. In a telegraph receiver, means for deriving rectified potentials corresponding to the intermittent marking elements of code signals, a pair of parallel-connected potential dividers in which a negative potential drop with respect to ground is built up by saidrectied potentials, a capacitor having a ground connection on one side andtwo parallel-connected, but oppositely faced unilateral conductances in circuit between the other side of said capacitor and a point intermediate the terminals of the rst of said potential dividers, means including a rst one of said unilateral conducta-noesfor building up a charge in said capacitor in response to the reception of a relatively strong marking signal, a limiter discharge tube having an input circuit connected from its grounded cathode through a portion of the second of said potential dividers Yto the control grid of said tube, whereby this tube is substantially blocked by a rectified signal potential across the second of said potential dividers,4 means conf trolled by said limiter tube, when it is blocked, for reducing the impedance of a second one of said unilateral conductances thereby to provide a discharge path for an excessive charge on said capacitor in the presence of a relatively weak marking signal, said means being also operative during a spacing signal condition for substantially blocking the flow of current through said second unilateral conductance, and means operative in accordance with the charge stored in said capacitor for regulating the gain in said receiver.

5. In an automatic gain control device for a radio telegraph receiver, a gain control circuit including a capacitor for storing a suitable bias potential with respect to ground, a detector for producing a unidirectional current in response` to the reception of each marking element of the Vcode signals, means including a circuit through 'tude limiter device controllable by a rectified out put component of said detector, and means for varying the impedance of said second unilateral conductance inversely -with respect to the space path impedance of'said amplitude limiter,

the last said means being effective to hold a- Vgiven charge in said capacitor for the duration of a normal spacing interval.

VPdLiYlviolvD A. WOOD.` 

